Wafer processing method

ABSTRACT

A wafer processing method includes a polyolefin sheet providing step of positioning a wafer in an inside opening of a ring frame and providing a polyolefin sheet on a back side of the wafer and on a back side of the ring frame, a uniting step of heating the polyolefin sheet as applying a pressure to the polyolefin sheet to thereby unite the wafer and the ring frame through the polyolefin sheet by thermocompression bonding, a dividing step of applying a laser beam to the wafer to form division grooves in the wafer, thereby dividing the wafer into individual device chips, and a pickup step of picking up each device chip from the polyolefin sheet.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a wafer processing method for dividinga wafer along a plurality of division lines to obtain a plurality ofindividual device chips, the division lines being formed on the frontside of the wafer to thereby define a plurality of separate regionswhere a plurality of devices are individually formed.

Description of the Related Art

In a fabrication process for device chips to be used in electronicequipment such as mobile phones and personal computers, a plurality ofcrossing division lines (streets) are first set on the front side of awafer formed of a semiconductor, for example, thereby defining aplurality of separate regions on the front side of the wafer. In eachseparate region, a device such as an integrated circuit (IC), alarge-scale integrated circuit (LSI), and a light emitting diode (LED)is next formed. Thereafter, a ring frame having an inside opening isprepared, in which an adhesive tape called a dicing tape is previouslyattached in its peripheral portion to the ring frame (the back side ofthe ring frame) so as to close the inside opening of the ring frame.Thereafter, a central portion of the adhesive tape is attached to theback side of the wafer such that the wafer is accommodated in the insideopening of the ring frame. In this manner, the wafer, the adhesive tape,and the ring frame are united together to form a frame unit. Thereafter,the wafer included in this frame unit is processed to be divided alongeach division line, thereby obtaining a plurality of individual devicechips including the respective devices.

For example, a laser processing apparatus is used to divide the wafer(see Japanese Patent Laid-Open No. H10-305420). The laser processingapparatus includes a chuck table for holding the wafer through theadhesive tape and a laser processing unit for applying a laser beam tothe wafer held on the chuck table, the laser beam having an absorptionwavelength to the wafer. In dividing the wafer by using this laserprocessing apparatus, the frame unit is placed on the chuck table, andthe wafer is held through the adhesive tape on the upper surface of thechuck table. In this condition, the chuck table and the laser processingunit are relatively moved in a direction parallel to the upper surfaceof the chuck table. At the same time, the laser beam is applied from thelaser processing unit to the wafer. When the laser beam is applied tothe wafer, laser ablation occurs to form a division groove in the waferalong each division line, thereby dividing the wafer along each divisionline.

Thereafter, the frame unit is transferred from the laser processingapparatus to another apparatus for applying ultraviolet light to theadhesive tape to thereby reduce the adhesion of the adhesive tape.Thereafter, each device chip is picked up from the adhesive tape. As aprocessing apparatus capable of producing the device chips with highefficiency, there is a processing apparatus capable of continuouslyperforming the operation for dividing the wafer and the operation forapplying ultraviolet light to the adhesive tape (see Japanese Patent No.3076179, for example). Each device chip picked up from the adhesive tapeis next mounted on a predetermined wiring substrate or the like.

SUMMARY OF THE INVENTION

The adhesive tape includes a base layer formed from a polyvinyl chloridesheet, for example, and an adhesive layer formed on the base layer. Inthe laser processing apparatus, the laser beam is applied to the waferunder the conditions where each division groove can be reliably formedin the wafer so as to have a depth from the front side to the back sideof the wafer, in order to reliably divide the wafer by laser ablation.As a result, the adhesive layer of the adhesive tape attached to theback side of the wafer is melted by the heat due to the application ofthe laser beam to the wafer at the position below or around eachdivision groove formed in the wafer, and a part of the adhesive layermelted is fixed to the back side of each device chip obtained from thewafer. In this case, in the step of picking up each device chip from theadhesive tape, ultraviolet light is applied to the adhesive tape toreduce the adhesion of the adhesive tape. However, the part of theadhesive layer melted and fixed to the back side of each device chipattached to the adhesive tape is yet left on the back side of eachdevice chip picked up from the adhesive tape. As a result, the qualityof each device chip is degraded.

The present invention has been made in view of such problems, and it istherefore an object of the present invention to provide a waferprocessing method which can prevent the adherence of the adhesive layerto the back side of each device chip obtained from a wafer, therebysuppressing a degradation in quality of each device chip due to theadherence of the adhesive layer.

In accordance with an aspect of the present invention, there is provideda wafer processing method for dividing a wafer along a plurality ofdivision lines to obtain a plurality of individual device chips, thedivision lines being formed on a front side of the wafer to therebydefine a plurality of separate regions where a plurality of devices areindividually formed. The wafer processing method includes a ring framepreparing step of preparing a ring frame having an inside opening foraccommodating the wafer, a polyolefin sheet providing step ofpositioning the wafer in the inside opening of the ring frame andproviding a polyolefin sheet on a back side of the wafer and on a backside of the ring frame, a uniting step of heating the polyolefin sheetas applying a pressure to the polyolefin sheet after performing thepolyolefin sheet providing step, thereby uniting the wafer and the ringframe through the polyolefin sheet by thermocompression bonding to forma frame unit in a condition where the front side of the wafer and thefront side of the ring frame are exposed upward, a dividing step ofapplying a laser beam to the wafer along each division line, the laserbeam having an absorption wavelength to the wafer, after performing theuniting step, thereby forming a division groove in the wafer along eachdivision line to divide the wafer into the individual device chips, anda pickup step of picking up each device chip from the polyolefin sheetafter performing the dividing step.

Preferably, the uniting step includes a step of applying infrared lightto the polyolefin sheet, thereby performing the thermocompressionbonding.

Preferably, the polyolefin sheet is larger in size than the ring frame,and the uniting step includes an additional step of cutting thepolyolefin sheet after heating the polyolefin sheet, thereby removing apart of the polyolefin sheet outside the outer circumference of the ringframe.

Preferably, the pickup step includes a first step of expanding thepolyolefin sheet to thereby increase a spacing between any adjacent onesof the device chips and a second step of pushing up each device chipthrough the polyolefin sheet after performing the first step.

Preferably, the polyolefin sheet is formed of a material selected fromthe group consisting of polyethylene, polypropylene, and polystyrene.

In the case that the polyolefin sheet is formed of polyethylene, thepolyolefin sheet is preferably heated in the range of 120° C. to 140° C.in the uniting step. In the case that the polyolefin sheet is formed ofpolypropylene, the polyolefin sheet is preferably heated in the range of160° C. to 180° C. in the uniting step. In the case that the polyolefinsheet is formed of polystyrene, the polyolefin sheet is preferablyheated in the range of 220° C. to 240° C. in the uniting step.

Preferably, the wafer is formed of a material selected from the groupconsisting of silicon, gallium nitride, gallium arsenide, and glass.

In the wafer processing method according to a preferred embodiment ofthe present invention, the wafer and the ring frame are united by usingthe polyolefin sheet having no adhesive layer in place of an adhesivetape having an adhesive layer, thereby forming the frame unit composedof the wafer, the ring frame, and the polyolefin sheet united together.The uniting step of uniting the wafer and the ring frame through thepolyolefin sheet is realized by thermocompression bonding. Afterperforming the uniting step, a laser beam having an absorptionwavelength to the wafer is applied to the wafer to thereby form adivision groove in the wafer along each division line by laser ablation,so that the wafer is divided along each division line to obtainindividual device chips attached to the polyolefin sheet. Thereafter,each device chip is picked up from the polyolefin sheet. Each devicechip picked up is next mounted on a predetermined mounting substrate orthe like.

In performing laser ablation to the wafer, the heat due to theapplication of the laser beam to the wafer is transmitted to thepolyolefin sheet at the position below or near each division groove.However, since the polyolefin sheet has no adhesive layer, there is noproblem that the adhesive layer may be melted to be fixed to the backside of each device chip. That is, according to one aspect of thepresent invention, the frame unit can be formed by using the polyolefinsheet having no adhesive layer, so that an adhesive tape having anadhesive layer is not required. As a result, it is possible to preventthe problem that the quality of each device chip is degraded by theadherence of the adhesive layer to each device chip.

Thus, the wafer processing method according to one aspect of the presentinvention can exhibit the effect that the adhesive layer does not adhereto the back side of each device chip obtained from the wafer, therebysuppressing a degradation in quality of each device chip due to theadherence of the adhesive layer.

The above and other objects, features, and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings depicting a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of a wafer;

FIG. 2 is a schematic perspective view depicting a manner of positioningthe wafer and a ring frame on a holding surface of a chuck table;

FIG. 3 is a schematic perspective view depicting a polyolefin sheetproviding step;

FIG. 4 is a schematic perspective view depicting a uniting step;

FIG. 5 is a schematic perspective view depicting a modification of theuniting step;

FIG. 6 is a schematic perspective view depicting another modification ofthe uniting step;

FIG. 7A is a schematic perspective view depicting a manner of cuttingthe polyolefin sheet after performing the uniting step;

FIG. 7B is a schematic perspective view of a frame unit formed byperforming the step depicted in FIG. 7A;

FIG. 8 is a schematic perspective view depicting a dividing step;

FIG. 9 is a schematic perspective view depicting a manner of loading theframe unit to a pickup apparatus after performing the dividing step;

FIG. 10A is a schematic sectional view depicting a standby conditionwhere the frame unit is fixed to a frame support table set at an initialposition in a pickup step using the pickup apparatus depicted in FIG. 9;and

FIG. 10B is a schematic sectional view depicting a working conditionwhere the frame support table holding the frame unit with the polyolefinsheet is lowered to expand the polyolefin sheet in the pickup step.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will now be describedwith reference to the attached drawings. There will first be described awafer to be processed by a wafer processing method according to thispreferred embodiment. FIG. 1 is a schematic perspective view of a wafer1. The wafer 1 is a substantially disc-shaped substrate formed of amaterial such as silicon (Si), silicon carbide (SiC), gallium nitride(GaN), and gallium arsenide (GaAs). The wafer 1 may be formed of anyother semiconductor materials. Further, the wafer 1 may be formed of amaterial such as sapphire, glass, and quartz. Examples of the glassinclude alkaline glass, nonalkaline glass, soda lime glass, lead glass,borosilicate glass, and silica glass. The wafer 1 has a front side 1 aand a back side 1 b. A plurality of crossing division lines 3 are formedon the front side 1 a of the wafer 1 to thereby define a plurality ofrespective separate regions where a plurality of devices 5 such as ICs,LSIs, and LEDs are formed. The crossing division lines 3 are composed ofa plurality of parallel division lines 3 extending in a first directionand a plurality of parallel division lines 3 extending in a seconddirection perpendicular to the first direction. In the processing methodfor the wafer 1 according to this preferred embodiment, laser ablationis performed to form a plurality of crossing division grooves along therespective crossing division lines 3 in the wafer 1, thereby dividingthe wafer 1 into a plurality of individual device chips each includingthe device 5.

The laser ablation is performed by using a laser processing apparatus 12(see FIG. 8). Prior to loading the wafer 1 into the laser processingapparatus 12, the wafer 1 is united with a polyolefin sheet 9 (see FIG.3) and a ring frame 7 (see FIG. 2) to thereby form a frame unit 11 (seeFIG. 8). Thus, the wafer 1 is loaded in the form of the frame unit 11into the laser processing apparatus 12 and then processed by the laserprocessing apparatus 12 to obtain the individual device chips, in whicheach device chip is supported to the polyolefin sheet 9. Thereafter, thepolyolefin sheet 9 is expanded to thereby increase the spacing betweenany adjacent ones of the device chips. Thereafter, each device chip ispicked up by using a pickup apparatus. The ring frame 7 is formed of arigid material such as metal, and it has a circular inside opening 7 ahaving a diameter larger than that of the wafer 1. The outside shape ofthe ring frame 7 is substantially circular. The ring frame 7 has a frontside 7 b and a back side 7 c. In forming the frame unit, the wafer 1 isaccommodated in the inside opening 7 a of the ring frame 7 andpositioned in such a manner that the center of the wafer 1 substantiallycoincides with the center of the inside opening 7 a.

The polyolefin sheet 9 is a flexible (expandable) resin sheet, and ithas a flat front side and a flat back side. The polyolefin sheet 9 is acircular sheet having a diameter larger than the outer diameter of thering frame 7. The polyolefin sheet 9 has no adhesive layer. Thepolyolefin sheet 9 is a sheet of a polymer (polyolefin) synthesized bypolymerizing alkene as a monomer. Examples of the polyolefin sheet 9include a polyethylene sheet, polypropylene sheet, and polystyrenesheet. The polyolefin sheet 9 is transparent or translucent to visiblelight. As a modification, the polyolefin sheet 9 may be opaque. Sincethe polyolefin sheet 9 has no adhesive property, it cannot be attachedto the wafer 1 and the ring frame 7 at room temperature. However, thepolyolefin sheet 9 is a thermoplastic sheet, so that, when thepolyolefin sheet 9 is heated to a temperature near its melting pointunder a predetermined pressure in a condition where the polyolefin sheet9 is in contact with the wafer 1 and the ring frame 7, the polyolefinsheet 9 is partially melted and thereby bonded to the wafer 1 and thering frame 7. That is, by applying heat and pressure to the polyolefinsheet 9 in the condition where the polyolefin sheet 9 is in contact withthe wafer 1 and the ring frame 7, the polyolefin sheet 9 can be bondedto the wafer 1 and the ring frame 7. Thusly, in the processing methodfor the wafer 1 according to this preferred embodiment, all of the wafer1, the ring frame 7, and the polyolefin sheet 9 are united bythermocompression bonding as mentioned above, thereby forming the frameunit.

The steps of the processing method for the wafer 1 according to thispreferred embodiment will now be described. Prior to uniting the wafer1, the polyolefin sheet 9, and the ring frame 7, a polyolefin sheetproviding step is performed by using a chuck table 2 having a holdingsurface 2 a depicted in FIG. 2. FIG. 2 is a schematic perspective viewdepicting a manner of positioning the wafer 1 and the ring frame 7 onthe holding surface 2 a of the chuck table 2. That is, the polyolefinsheet providing step is performed on the holding surface 2 a of thechuck table 2 as depicted in FIG. 2. The chuck table 2 has a circularporous member having a diameter larger than the outer diameter of thering frame 7. The porous member constitutes a central upper portion ofthe chuck table 2. The porous member has an upper surface functioning asthe holding surface 2 a of the chuck table 2. A suction passage (notdepicted) is formed in the chuck table 2, in which one end of thesuction passage is connected to the porous member. Further, a vacuumsource 2 b (see FIG. 3) is connected to the other end of the suctionpassage. The suction passage is provided with a selector 2 c (see FIG.3) for switching between an ON condition and an OFF condition. When theON condition is established by the selector 2 c, a vacuum produced bythe vacuum source 2 b is applied to a workpiece placed on the holdingsurface 2 a of the chuck table 2, thereby holding the workpiece on thechuck table 2 under suction.

In the polyolefin sheet providing step, the wafer 1 and the ring frame 7are first placed on the holding surface 2 a of the chuck table 2 asdepicted in FIG. 2. At this time, the front side 1 a of the wafer 1 isoriented downward, and the front side 7 b of the ring frame 7 is alsooriented downward. In this condition, the wafer 1 is positioned in theinside opening 7 a of the ring frame 7. Thereafter, as depicted in FIG.3, the polyolefin sheet 9 is provided on the back side 1 b (uppersurface) of the wafer 1 and on the back side 7 c (upper surface) of thering frame 7. FIG. 3 is a schematic perspective view depicting a mannerof providing the polyolefin sheet 9 on the wafer 1 and the ring frame 7.That is, as depicted in FIG. 3, the polyolefin sheet 9 is provided so asto fully cover the wafer 1 and the ring frame 7. In the polyolefin sheetproviding step, the diameter of the polyolefin sheet 9 is set largerthan the diameter of the holding surface 2 a of the chuck table 2.Unless the diameter of the polyolefin sheet 9 is larger than thediameter of the holding surface 2 a, there may arise a problem suchthat, when the vacuum from the vacuum source 2 b is applied to theholding surface 2 a of the chuck table 2 in a uniting step to beperformed later, the vacuum may leak from any gap between the polyolefinsheet 9 and the holding surface 2 a because the holding surface 2 a isnot fully covered with the polyolefin sheet 9, so that a pressure cannotbe properly applied to the polyolefin sheet 9.

In the processing method for the wafer 1 according to this preferredembodiment, a uniting step is next performed in such a manner that thepolyolefin sheet 9 is heated to unite the wafer 1 and the ring frame 7through the polyolefin sheet 9 by thermocompression bonding. FIG. 4 is aschematic perspective view depicting the uniting step according to thispreferred embodiment. As depicted in FIG. 4, the polyolefin sheet 9transparent or translucent to visible light is provided so as to fullycover the wafer 1, the ring frame 7, and the holding surface 2 a of thechuck table 2, which are all depicted by broken lines in FIG. 4. In theuniting step, the selector 2 c is operated to establish the ON conditionwhere the vacuum source 2 b is in communication with the porous memberof the chuck table 2, i.e., the holding surface 2 a of the chuck table2, so that a vacuum produced by the vacuum source 2 b is applied to thepolyolefin sheet 9 provided on the chuck table 2. Accordingly, thepolyolefin sheet 9 is brought into close contact with the wafer 1 andthe ring frame 7 by the atmospheric pressure applied to the uppersurface of the polyolefin sheet 9.

Thereafter, the polyolefin sheet 9 is heated in a condition where thepolyolefin sheet 9 is sucked by the vacuum source 2 b, therebyperforming thermocompression bonding. In this preferred embodimentdepicted in FIG. 4, for example, the heating of the polyolefin sheet 9is effected by a heat gun 4 provided above the chuck table 2. The heatgun 4 includes heating means such as a heating wire and an air blowingmechanism such as a fan. Accordingly, the heat gun 4 can heat ambientair and blow the heated air. In a condition where the vacuum from thevacuum source 2 b is applied to the polyolefin sheet 9, the heat gun 4is operated to supply hot air 4 a to the upper surface of the polyolefinsheet 9. Accordingly, when the polyolefin sheet 9 is heated to apredetermined temperature, the polyolefin sheet 9 is bonded to the wafer1 and the ring frame 7 by thermocompression bonding.

Another method for heating the polyolefin sheet 9 may be adopted. Forexample, any member heated to a predetermined temperature may be pressedon the polyolefin sheet 9 against the wafer 1 and the ring frame 7. FIG.5 is a schematic perspective view depicting such a modification of theuniting step. As depicted in FIG. 5, the polyolefin sheet 9 transparentor translucent to visible light is provided so as to fully cover thewafer 1, the ring frame 7, and the holding surface 2 a of the chucktable 2, which are all depicted by broken lines in FIG. 5. In thismodification depicted in FIG. 5, a heat roller 6 including a heat sourceis used. More specifically, the vacuum produced by the vacuum source 2 bis first applied to the polyolefin sheet 9, so that the polyolefin sheet9 is brought into close contact with the wafer 1 and the ring frame 7 bythe atmospheric pressure applied to the upper surface of the polyolefinsheet 9.

Thereafter, the heat roller 6 is heated to a predetermined temperature,and next placed on the holding surface 2 a of the chuck table 2 at oneend lying on the outer circumference of the holding surface 2 a asdepicted in FIG. 5. Thereafter, the heat roller 6 is rotated about itsaxis to roll on the chuck table 2 through the polyolefin sheet 9 fromthe above one end to another end diametrically opposite to the above oneend. As a result, the polyolefin sheet 9 is bonded to the wafer 1 andthe ring frame 7 by thermocompression bonding. In the case that a forcefor pressing the polyolefin sheet 9 is applied by the heat roller 6, thethermocompression bonding is effected at a pressure higher thanatmospheric pressure. Preferably, a cylindrical surface of the heatroller 6 is coated with fluororesin. Further, the heat roller 6 may bereplaced by any iron-like pressure member having a flat base plate andcontaining a heat source. In this case, the pressure member is heated toa predetermined temperature to thereby provide a hot plate, which isnext pressed on the polyolefin sheet 9 held on the chuck table 2.

Still another method for heating the polyolefin sheet 9 may be adoptedin the following manner. FIG. 6 is a schematic perspective viewdepicting such another modification of the uniting step. As depicted inFIG. 6, the polyolefin sheet 9 transparent or translucent to visiblelight is provided so as to fully cover the wafer 1, the ring frame 7,and the holding surface 2 a of the chuck table 2, which are all depictedby broken lines in FIG. 6. In this modification depicted in FIG. 6, aninfrared lamp 8 is provided above the chuck table 2 to heat thepolyolefin sheet 9. The infrared lamp 8 can apply infrared light 8 ahaving an absorption wavelength to at least the material of thepolyolefin sheet 9. Also in the modification depicted in FIG. 6, thevacuum produced by the vacuum source 2 b is first applied to thepolyolefin sheet 9, so that the polyolefin sheet 9 is brought into closecontact with the wafer 1 and the ring frame 7 by the atmosphericpressure applied to the upper surface of the polyolefin sheet 9.Thereafter, the infrared lamp 8 is operated to apply the infrared light8 a to the polyolefin sheet 9, thereby heating the polyolefin sheet 9.As a result, the polyolefin sheet 9 is bonded to the wafer 1 and thering frame 7 by thermocompression bonding.

When the polyolefin sheet 9 is heated to a temperature near its meltingpoint by performing any one of the above methods, the polyolefin sheet 9is bonded to the wafer 1 and the ring frame 7 by thermocompressionbonding. After bonding the polyolefin sheet 9, the selector 2 c isoperated to establish the OFF condition where the communication betweenthe porous member of the chuck table 2 and the vacuum source 2 b iscanceled. Accordingly, the suction holding by the chuck table 2 iscanceled.

Thereafter, the polyolefin sheet 9 is circularly cut along the outercircumference of the ring frame 7 to remove an unwanted peripheralportion of the polyolefin sheet 9. FIG. 7A is a schematic perspectiveview depicting a manner of cutting the polyolefin sheet 9. As depictedin FIG. 7A, a disc-shaped (annular) cutter 10 is used to cut thepolyolefin sheet 9. The cutter 10 has a central through hole 10 a inwhich a rotating shaft 10 b is fitted. Accordingly, the cutter 10 isrotatable about the axis of the rotating shaft 10 b. First, the cutter10 is positioned above the ring frame 7. At this time, the rotatingshaft 10 b is set so as to extend in the radial direction of the chucktable 2. Thereafter, the cutter 10 is lowered until the outercircumference (cutting edge) of the cutter 10 comes into contact withthe polyolefin sheet 9 placed on the ring frame 7. That is, thepolyolefin sheet 9 is caught between the cutter 10 and the ring frame 7,so that the polyolefin sheet 9 is cut by the cutter 10 to form a cutmark 9 a. Further, the cutter 10 is rolled on the polyolefin sheet 9along a circular line set between the inner circumference of the ringframe 7 (i.e., the periphery of the inside opening 7 a of the ring frame7) and the outer circumference of the ring frame 7, thereby circularlyforming the cut mark 9 a along the above circular line. As a result, apredetermined central portion of the polyolefin sheet 9 is surrounded bythe circular cut mark 9 a. Thereafter, a remaining peripheral portion ofthe polyolefin sheet 9 outside the circular cut mark 9 a is removed.That is, an unwanted peripheral portion of the polyolefin sheet 9including an outermost peripheral portion outside the outercircumference of the ring frame 7 can be removed.

The cutter 10 may be replaced by an ultrasonic cutter for cutting thepolyolefin sheet 9. Further, a vibration source for vibrating the cutter10 at a frequency in an ultrasonic band may be connected to the cutter10. Further, in cutting the polyolefin sheet 9, the polyolefin sheet 9may be cooled to be hardened in order to facilitate the cuttingoperation. By cutting the polyolefin sheet 9 as mentioned above, a frameunit 11 depicted in FIG. 7B is formed, in which the frame unit 11 iscomposed of the wafer 1, the ring frame 7, and the polyolefin sheet 9united together. That is, the wafer 1 and the ring frame 7 are unitedwith each other through the polyolefin sheet 9 to form the frame unit 11as depicted in FIG. 7B. FIG. 7B is a schematic perspective view of theframe unit 11 in a condition where the front side 1 a of the wafer 1 andthe front side 7 b of the ring frame 7 are exposed upward.

In performing the thermocompression bonding as mentioned above, thepolyolefin sheet 9 is heated preferably to a temperature lower than orequal to the melting point of the polyolefin sheet 9. If the heatingtemperature is higher than the melting point of the polyolefin sheet 9,there is a possibility that the polyolefin sheet 9 may be melted to suchan extent that the shape of the polyolefin sheet 9 cannot be maintained.Further, the polyolefin sheet 9 is heated preferably to a temperaturehigher than or equal to the softening point of the polyolefin sheet 9.If the heating temperature is lower than the softening point of thepolyolefin sheet 9, the thermocompression bonding cannot be properlyperformed. Accordingly, the polyolefin sheet 9 is heated preferably to atemperature higher than or equal to the softening point of thepolyolefin sheet 9 and lower than or equal to the melting point of thepolyolefin sheet 9. Further, there is a case that the softening point ofthe polyolefin sheet 9 may be unclear. To cope with such a case, inperforming the thermocompression bonding, the polyolefin sheet 9 isheated preferably to a temperature higher than or equal to a presettemperature and lower than or equal to the melting point of thepolyolefin sheet 9, the preset temperature being lower by 20° C. thanthe melting point of the polyolefin sheet 9.

In the case that the polyolefin sheet 9 is a polyethylene sheet, theheating temperature in the uniting step is preferably set in the rangeof 120° C. to 140° C. Further, in the case that the polyolefin sheet 9is a polypropylene sheet, the heating temperature in the uniting step ispreferably set in the range of 160° C. to 180° C. Further, in the casethat the polyolefin sheet 9 is a polystyrene sheet, the heatingtemperature in the uniting step is preferably set in the range of 220°C. to 240° C.

The heating temperature is defined herein as the temperature of thepolyolefin sheet 9 to be heated in performing the uniting step. As theheat sources included in the heat gun 4, the heat roller 6, and theinfrared lamp 8 mentioned above, some kind of heat source capable ofsetting an output temperature has been put into practical use. However,even when such a heat source is used to heat the polyolefin sheet 9, thetemperature of the polyolefin sheet 9 does not reach the outputtemperature set above in some case. To cope with such a case, the outputtemperature of the heat source may be set to a temperature higher thanthe melting point of the polyolefin sheet 9 in order to heat thepolyolefin sheet 9 to a predetermined temperature.

After performing the uniting step mentioned above, a dividing step isperformed in such a manner that the wafer 1 in the condition of theframe unit 11 is processed by laser ablation to form a plurality ofcrossing division grooves along the plural crossing division lines 3 inthe wafer 1, thereby dividing the wafer 1 into individual device chips.The dividing step is performed by using a laser processing apparatus 12depicted in FIG. 8 in this preferred embodiment. FIG. 8 is a schematicperspective view depicting the dividing step. As depicted in FIG. 8, thelaser processing apparatus 12 includes a laser processing unit 14 forperforming laser ablation to the wafer 1 and a chuck table (notdepicted) for holding the wafer 1. The laser processing unit 14 includesa laser oscillator (not depicted) for generating a laser beam 16 havingan absorption wavelength to the wafer 1 (having a wavelength absorptiveto the wafer 1). The chuck table has an upper surface as a holdingsurface for holding the wafer 1. The chuck table is movable in adirection parallel to the upper surface thereof, that is, movable in afeeding direction. The laser beam 16 generated from the laser oscillatorin the laser processing unit 14 is applied to the wafer 1 held on thechuck table. The laser processing unit 14 further includes a processinghead 14 a having a mechanism for focusing the laser beam 16 at apredetermined vertical position in the wafer 1.

In performing laser ablation to the wafer 1, the frame unit 11 is placedon the chuck table in the condition where the front side 1 a of thewafer 1 is exposed upward. Accordingly, the wafer 1 is held through thepolyolefin sheet 9 on the chuck table. Thereafter, the chuck table isrotated to make the division lines 3 extending in the first direction onthe front side 1 a of the wafer 1 parallel to a feeding direction in thelaser processing apparatus 12. Further, the chuck table and the laserprocessing unit 14 are relatively moved to adjust a relative position,thereby positioning the processing head 14 a directly above an extensionof a predetermined one of the division lines 3 extending in the firstdirection. Thereafter, the laser beam 16 is applied from the processinghead 14 a to the wafer 1. At the same time, the chuck table and thelaser processing unit 14 are relatively moved in the feeding directionparallel to the upper surface of the chuck table. As a result, the laserbeam 16 is applied to the wafer 1 along the predetermined division line3, thereby performing laser ablation along the predetermined divisionline 3. Accordingly, a division groove 3 a is formed in the wafer 1along the predetermined division line 3 by the laser beam 16. Thedivision groove 3 a has a depth from the front side 1 a to the back side1 b of the wafer 1. In this dividing step, the laser beam 16 may beapplied under the following processing conditions. The followingprocessing conditions are merely illustrative.

Wavelength: 355 nm

Repetition frequency: 50 kHz

Average power: 5 W

Feed speed: 200 mm/s

After forming the division groove 3 a along the predetermined divisionline 3, the chuck table and the laser processing unit 14 are moved in anindexing direction perpendicular to the feeding direction to similarlyperform laser ablation along the next division line 3 extending in thefirst direction. Thereafter, laser ablation is similarly performed alongall of the other division lines 3 extending in the first direction.Thus, a plurality of similar division grooves 3 a are formed along allof the division lines 3 extending in the first direction. Thereafter,the chuck table is rotated 90 degrees about its vertical axisperpendicular to the holding surface thereof to similarly perform laserablation along all of the division lines 3 extending in the seconddirection perpendicular to the first direction. Thus, a plurality ofsimilar division grooves 3 a are formed along all of the division lines3 extending in the second direction. In this manner, the plural crossingdivision grooves 3 a having a depth from the front side 1 a to the backside 1 b of the wafer 1 are formed in the wafer 1 along the respectivecrossing division lines 3, so that the wafer 1 is divided by thesedivision lines 3 a to obtain the individual device chips.

When the laser processing unit 14 is operated to perform laser ablationto the wafer 1, processing dust is generated from the wafer 1 at theposition where the laser beam 16 is applied. This processing dustscatters around this laser applying position to adhere to the front side1 a of the wafer 1. Although the front side 1 a of the wafer 1 iscleaned by using a cleaning unit to be hereinafter described afterperforming the laser ablation to the wafer 1, it is not easy tocompletely remove this processing dust adhering to the front side 1 a ofthe wafer 1. If the processing dust is left on each device chip obtainedfrom the wafer 1, the quality of each device chip is degraded. To copewith this problem, a water-soluble liquid resin may be previouslyapplied to the front side 1 a of the wafer 1, in which thiswater-soluble liquid resin functions as a protective film for protectingthe front side 1 a of the wafer 1. In the case that the liquid resin ispreviously applied to the front side 1 a of the wafer 1, the processingdust scattering in performing the laser ablation adheres to the uppersurface of the liquid resin applied. That is, the processing dust isprevented from directly adhering to the front side 1 a of the wafer 1.After performing the laser ablation, the wafer 1 is cleaned by using thecleaning unit. At this time, the processing dust can be removed togetherwith the liquid resin applied in cleaning the wafer 1.

That is, the laser processing apparatus 12 may include such a cleaningunit (not depicted). In this case, the wafer 1 processed by the laserprocessing unit 14 as mentioned above is transferred to the cleaningunit and then cleaned by the cleaning unit. For example, the cleaningunit includes a cleaning table having a holding surface for holding theframe unit 11 and a cleaning water nozzle adapted to be horizontallymoved back and forth above the frame unit 11 held on the holding surfaceof the cleaning table. The cleaning water nozzle functions to supply acleaning water such as pure water to the wafer 1. The cleaning table isrotatable about its axis perpendicular to the holding surface thereof.In operation, the cleaning table is rotated about its axis and at thesame time the cleaning water is supplied from the cleaning water nozzleto the wafer 1. During this supply of the cleaning water, the cleaningwater nozzle is horizontally moved back and forth along a path passingthrough the position directly above the center of the holding surface ofthe cleaning table. Accordingly, the entire surface of the front side 1a of the wafer 1 can be cleaned by the cleaning water.

After performing the dividing step or the cleaning step, a pickup stepis performed to pick up each device chip from the polyolefin sheet 9.The pickup step is performed by using a pickup apparatus 18 depicted inFIG. 9. FIG. 9 is a schematic perspective view depicting a manner ofloading the frame unit 11 to the pickup apparatus 18. As depicted inFIG. 9, the pickup apparatus 18 includes a cylindrical drum 20 and aframe holding unit 22 having a frame support table 26 provided aroundthe cylindrical drum 20. The cylindrical drum 20 has an inner diameterlarger than the diameter of the wafer 1 and an outer diameter smallerthan the inner diameter of the ring frame 7 (the diameter of the insideopening 7 a). The frame support table 26 of the frame holding unit 22 isan annular table having a circular inside opening larger in diameterthan the drum 20. That is, the frame support table 26 has an innerdiameter larger than the outer diameter of the drum 20. Further, theframe support table 26 has an outer diameter larger than the outerdiameter of the ring frame 7. The inner diameter of the frame supporttable 26 is substantially equal to the inner diameter of the ring frame7. The frame support table 26 has an upper surface as a supportingsurface for supporting the ring frame 7 thereon through the polyolefinsheet 9. Initially, the height of the upper surface of the frame supporttable 26 is set equal to the height of the upper end of the drum 20 (seeFIG. 10A). Further, the upper end portion of the drum 20 is surroundedby the inner circumference of the ring frame 7 in this initial stage.

A plurality of clamps 24 are provided on the outer circumference of theframe support table 26. Each clamp 24 functions to hold the ring frame 7supported on the frame support table 26. That is, when the ring frame 7of the frame unit 11 is placed through the polyolefin sheet 9 on theframe support table 26 and then held by each clamp 24, the frame unit 11can be fixed to the frame support table 26. The frame support table 26is supported by a plurality of rods 28 extending in a verticaldirection. That is, each rod 28 is connected at its upper end to thelower surface of the frame support table 26. An air cylinder 30 forvertically moving each rod 28 is connected to the lower end of each rod28. More specifically, the lower end of each rod 28 is connected to apiston (not depicted) movably accommodated in the air cylinder 30. Eachair cylinder 30 is supported to a disc-shaped base 32. That is, thelower end of each air cylinder 30 is connected to the upper surface ofthe disc-shaped base 32. Accordingly, when each air cylinder 30 isoperated in the initial stage, the frame support table 26 is loweredwith respect to the drum 20 fixed in position.

Further, a pushup mechanism 34 for pushing up each device chip supportedto the polyolefin sheet 9 is provided inside the drum 20. That is, eachdevice chip is adapted to be pushed up through the polyolefin sheet 9 bythe pushup mechanism 34 located below the polyolefin sheet 9. Further, acollet 36 (see FIG. 10B) capable of holding each device chip undersuction is provided above the drum 20. Both the pushup mechanism 34 andthe collet 36 are movable in a horizontal direction parallel to theupper surface of the frame support table 26. The collet 36 is connectedthrough a selector 36 b (see FIG. 10B) to a vacuum source 36 a (see FIG.10B).

In the pickup step, each air cylinder 30 in the pickup apparatus 18 isfirst operated to adjust the height of the frame support table 26 suchthat the height of the upper end of the drum 20 becomes equal to theheight of the upper surface of the frame support table 26. Thereafter,the frame unit 11 transferred from the laser processing apparatus 12 isplaced on the drum 20 and the frame support table 26 in the pickupapparatus 18 in a condition where the front side 1 a of the wafer 1 ofthe frame unit 11 is oriented upward. Thereafter, each clamp 24 isoperated to fix the ring frame 7 of the frame unit 11 to the uppersurface of the frame support table 26. FIG. 10A is a schematic sectionalview depicting a standby condition where the frame unit 11 is fixed tothe frame support table 26 set at the initial position. At this time,the plural division grooves 3 a have already been formed in the wafer 1in the dividing step, so that the wafer 1 has already been divided intoa plurality of individual device chips 1 c (see FIG. 10B).

Thereafter, each air cylinder 30 is operated to lower the frame supporttable 26 of the frame holding unit 22 with respect to the drum 20. As aresult, the polyolefin sheet 9 fixed to the frame holding unit 22 byeach clamp 24 is expanded radially outward as depicted in FIG. 10B. FIG.10B is a schematic sectional view depicting a working condition wherethe frame support table 26 holding the ring frame 7 with the polyolefinsheet 9 is lowered to expand the polyolefin sheet 9. When the polyolefinsheet 9 is expanded radially outward as mentioned above, the spacingbetween any adjacent ones of the device chips 1 c supported to thepolyolefin sheet 9 is increased as depicted in FIG. 10B. Accordingly,the contact between the adjacent device chips 1 c can be suppressed, andeach device chip 1 c can be easily picked up. Thereafter, a target oneof the device chips 1 c is decided, and the pushup mechanism 34 is nextmoved to a position directly below this target device chip 1 c asdepicted in FIG. 10B. Furthermore, the collet 36 is also moved to aposition directly above this target device chip 1 c as depicted in FIG.10B. Thereafter, the pushup mechanism 34 is operated to push up thetarget device chip 1 c through the polyolefin sheet 9. Further, theselector 36 b is operated to make the collet 36 communicate with thevacuum source 36 a. As a result, the target device chip 1 c is heldunder suction by the collet 36 and thereby picked up from the polyolefinsheet 9. Such a pickup operation is similarly performed for all theother device chips 1 c. Thereafter, each device chip 1 c picked up ismounted on a predetermined wiring substrate or the like for actual use.

In the case of forming the frame unit 11 by using an adhesive tape, heatgenerated by the application of the laser beam 16 in the dividing stepis transmitted to the adhesive tape, so that the adhesive layer of theadhesive tape is melted and fixed to the back side of each device chip.Accordingly, in this case, there is a problem that the adherence of theadhesive layer to each device chip causes a degradation in quality. Tothe contrary, in the wafer processing method according to this preferredembodiment, the frame unit 11 can be formed by using the polyolefinsheet 9 having no adhesive layer, in which the polyolefin sheet 9 isattached to the wafer 1 and the ring frame 7 by thermocompressionbonding. That is, an adhesive tape having an adhesive layer is notrequired. As a result, it is possible to prevent a degradation inquality of each device chip due to the adherence of the adhesive layerto the back side of each device chip.

The present invention is not limited to the above preferred embodiment,but various modifications may be made within the scope of the presentinvention. For example, while the polyolefin sheet 9 is selected from apolyethylene sheet, a polypropylene sheet, and a polystyrene sheet inthe above preferred embodiment, this is merely illustrative. That is,the polyolefin sheet usable in the present invention may be formed ofany other materials (polyolefins) such as a copolymer of propylene andethylene and an olefin elastomer.

The present invention is not limited to the details of the abovedescribed preferred embodiment. The scope of the invention is defined bythe appended claims and all changes and modifications as fall within theequivalence of the scope of the claims are therefore to be embraced bythe invention.

What is claimed is:
 1. A wafer processing method for dividing a waferalong a plurality of division lines to obtain a plurality of individualdevice chips, the division lines being formed on the front side of thewafer to thereby define a plurality of separate regions where aplurality of devices are individually formed, the wafer processingmethod comprising: a ring frame preparing step of preparing a ring framehaving an inside opening for accommodating the wafer; a polyolefin sheetproviding step of positioning the wafer in the inside opening of thering frame and providing a polyolefin sheet on a back side of the waferand on a back side of the ring frame; a uniting step of heating thepolyolefin sheet as applying a pressure to the polyolefin sheet afterperforming the polyolefin sheet providing step, thereby uniting thewafer and the ring frame through the polyolefin sheet bythermocompression bonding to form a frame unit in a condition where thefront side of the wafer and the front side of the ring frame are exposedupward; a dividing step of applying a laser beam to the wafer along eachdivision line, the laser beam having an absorption wavelength to thewafer, after performing the uniting step, thereby forming a divisiongroove in the wafer along each division line to divide the wafer intothe individual device chips; and a pickup step of picking up each devicechip from the polyolefin sheet after performing the dividing step. 2.The wafer processing method according to claim 1, wherein the unitingstep includes a step of applying infrared light to the polyolefin sheet,thereby performing the thermocompression bonding.
 3. The waferprocessing method according to claim 1, wherein the polyolefin sheet islarger in size than the ring frame, and the uniting step includes anadditional step of cutting the polyolefin sheet after heating thepolyolefin sheet, thereby removing a part of the polyolefin sheetoutside an outer circumference of the ring frame.
 4. The waferprocessing method according to claim 1, wherein the pickup step includesa first step of expanding the polyolefin sheet to thereby increase aspacing between any adjacent ones of the device chips and a second stepof pushing up each device chip through the polyolefin sheet afterperforming the first step.
 5. The wafer processing method according toclaim 1, wherein the polyolefin sheet is formed of a material selectedfrom the group consisting of polyethylene, polypropylene, andpolystyrene.
 6. The wafer processing method according to claim 5,wherein the polyolefin sheet is formed of polyethylene, and thepolyolefin sheet is heated in the range of 120° C. to 140° C. in theuniting step.
 7. The wafer processing method according to claim 5,wherein the polyolefin sheet is formed of polypropylene, and thepolyolefin sheet is heated in the range of 160° C. to 180° C. in theuniting step.
 8. The wafer processing method according to claim 5,wherein the polyolefin sheet is formed of polystyrene, and thepolyolefin sheet is heated in the range of 220° C. to 240° C. in theuniting step.
 9. The wafer processing method according to claim 1,wherein the wafer is formed of a material selected from the groupconsisting of silicon, gallium nitride, gallium arsenide, and glass.